In various processes requiring the filtration of a gas or liquid stream, filters composed of woven or nonwoven fibers are often employed, either alone in flat sheet or tubular form or supported by a suitable porous support. In the selection of such filters for a particular application, one characteristic relates to the efficiency, flow rate and life of the filter. These criteria typically depend upon the particular properties of the material of which the fiber is composed, but more particularly, are related to the particular diameter of the fibers used. Another important criteria is an environmental one as to whether the filters will withstand the particular pressures, temperatures and physical nature of the gas or liquid material to be filtered.
As to this latter criteria, filters made from organic material are quite susceptible to pressure, temperature and the chemical nature of the gas or liquid to be filtered. Filters made from glass fibers have been employed where the glass fibers are formed into an interlaced mass of the glass fibers without any physical coherency or strength; that is, without a binder to retain the glass fibers in a coherent manner. Although such glass fibers have good temperature and chemical resistance, the physical strength of such fibers is quite low, and such fibers may only be used in most undemanding applications. The filters must, therefore, be treated with a great deal of care by the user in order that they remain as useful filters.
The lack of strength of such glass fibers has been overcome by bonding the glass fibers with suitable organic bonding agents. The bonding agents may typically be phenol-formaldehyde or epoxy resins or other thermosetting-type resins with which the mass of typically interrelated nonwoven glass fibers are impregnated during the formation process of the fibers into their particular form or thereafter. However, the chemical and temperature resistance of the filter so prepared is modified by the employment of such bonding agents. Typically an epoxy resin is used which provides good strength and fairly high-temperature and chemical resistance to a self-supporting nonwoven tubular fiber filter. Such filter tubes are described more particularly in U.S. Pat. No. 3,767,054, issued Oct. 23, 1973. Although such epoxy resin glass fiber filter tubes are suitable for many uses, the organic bonding agent restricts the scope of the filter applications available, particularly since the maximum temperature of use of such tubes is not over 200.degree. C. In addition, such filter tubes are not very resistant to degradation in use with many liquids, such as concentrated acids. Furthermore, such filter tubes, due to the presence of the organic resinous bonding agent, often have an off-white-to-light-brown color which darkens with age and sunlight due to the presence of such bonding agent. Thus, such filters may not be employed where a color indicator is employed with the filter, or where a white color is desirable.